This invention relates generally to environmental control systems for aircraft, and more particularly to a novel aircraft fuel-based heat management system for controlling aircraft engine fuel temperature and for providing heat rejection capability for heat-generating aircraft systems.
Conventional environmental control systems for aircraft rely to a significant degree upon heat exchange with the atmosphere in order to achieve cooling. The reliance upon heat exchange with ambient constrains the operation of such environmental control system to aircraft speed and altitude limits within which the atmosphere presents a practical heat sink for that system. Further, the ingestion into occupied aircraft compartments of ambient atmospheric air may be undesirable in certain instances involving an environment contaminated with airborn chemical or bacteriological agents. The existence of air inlets and outlets on the aircraft, particularly associated with heat rejection, also detracts from the stealthiness of the aircraft. Certain existing heat exchange systems may be configured to rely partially upon heat exchange with the fuel, but these systems are capable of servicing relatively small heat loads thereby, and are wholly inadequate for meeting total long-range aircraft mission heat rejection requirements without substantial reliance upon exchange with ambient.
The novel heat management system for aircraft disclosed herein provides an onboard heat rejection system which may be based exclusively upon the use of an expendable liquid, such as aircraft fuel, which is capable of storing thermal energy. In a preferred embodiment of this invention heat exchange between on-board heat generating systems and aircraft fuel may be accomplished by incorporating into a uniquely configured fuel loop a dedicated fuel tank for storing fuel which is cooled by heat exchange with the aircraft primary fuel flow. The novel system of this invention may have the heat management capacity sufficient to avoid the need for any auxiliary ambient air based source of cooling for the aircraft.
The system described herein may include two separate cooling configurations, operable alternately or in combination, to manage the heat sink capacity of the aircraft fuel, and, simultaneously, to maintain the engine fuel temperature below a maximum desirable operating limit. First, the flow rate through the primary engine fuel line may be maintained above that required for consumption by the engines, and the excess may be recirculated to a main fuel tank. Second, a dedicated fuel tank chiller loop which rejects heat to the primary fuel flow, may refrigerate fuel stored in the dedicated fuel tank at levels which may permit completion of a mission without exceeding the maximum desirable engine fuel temperature and without the use of supplementary cooling systems.
The components of the system of the present invention comprise a fuel line loop representing the cooling cycle coupled on its low temperature side to a dedicated thermal reserve fuel tank and on its high temperature side to the fuel stream to the aircraft engine(s); a microcomputer based controller monitors the fuel line flow and temperature at a plurality of locations throughout the system and controls the fuel flow both to the engine and through the recirculation loop; one or more heat exchange subsystems using the fuel flow as a heat exchange medium may be included to maintain various aircraft systems at temperatures consistent with the schedule of aircraft heat generation profiles and mission cooling requirements.
The controlling parameter for operation of the heat management system of this invention is the temperature of the aircraft fuel entering the fuel line(s) to the engine(s). The objective of the heat management system of this invention is to reject substantially all heat loads of the aircraft systems to the fuel stream while maintaining the fuel temperature typically below a specified upper design limit. Depending upon the aircraft, engine type, and fuel system design, this temperature may typically be in the range of from about 140.degree. F. (60.degree. C.) to about 300.degree. F. (149.degree. C.), the maximum allowable upper limit being related to the coking temperature of the fuel at which point the risk of fouling engine fuel lines and other fuel supply components becomes significant, or being related to the cavitation limits of the fuel pumps. Further, during certain aircraft mission phases, the combination of fuel temperature and flow rate may result in the maximum allowable fuel temperature being exceeded when all aircraft heat loads are rejected to the fuel stream, but during other mission phases the fuel may assume a temperature well below the maximum desirable limit. Therefore, during aircraft operating periods during which the fuel temperature is below the said limit the active cooling cycle of the novel heat management system of this invention is functional to cool the fuel of the dedicated thermal reserve fuel tank, which, during periods of undesirable fuel temperature excursions, may, in an alternative functional embodiment hereof, provide a source of chilled fuel to blend with the main fuel flow to the engine to ensure that the maximum allowable fuel temperature limit is not exceeded.
The novel heat management system of the present invention, in providing a system for absorbing aircraft heat loads during an entire mission without the use of an ambient air based auxiliary system, lowers aircraft observability by substantially eliminating air inlets and outlets associated with or characterizing ambient based environmental control systems.
Aircraft operation reliability is improved by providing nearly constant temperature heat sink for cooling the aircraft avionics systems. Fuel consumption associated with environmental control systems operation is significantly reduced, resulting in fuel savings and extended aircraft range.
It is, therefore, an object of this invention to provide an improved heat management system for aircraft.
It is a further object of this invention to provide an improved fuel-based cooling system for heat generating aircraft systems.
These and other objects of the present invention will become apparent as the description of representative embodiments proceeds.